<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<h3>
    结果：<span></span>
</h3>
<script>
    let i1=document.querySelector("#i1");
    let i2=document.querySelector("#i2");
    let s=document.querySelector("span")
    function f(x){
        if (isNaN(i1.value||i2.value)){
            s.innerText = "输入错误!";
            return;
        }
       switch (x){
             case 1: s.innerText=i1.value*1+i2.value*1;break;
             case 2:s.innerText=i1.value*1-i2.value*1;break;
             case 3:s.innerText=i1.value*i2.value;break;
             case 4:s.innerText=i1.value/i2.value;break;
        }

    }


</script>
</body>
</html>